An underwater pipeline is normally assembled on board a laying vessel, and laid on the bed of a body of water as it is assembled. Some commonly used laying vessels are equipped with a substantially horizontal, on-board assembly line; and a curved lay ramp that guides part of the pipeline as it is lowered onto the bed. This laying method is known as S-laying, because of the shape of the pipeline between the laying vessel and the bed.
On other laying vessels, the pipeline is assembled in a substantially vertical tower and released substantially vertically. This laying method is known as J-laying, because of the shape of the pipeline between the laying vessel and the bed, and is preferable to S-laying when working in deep water.
Substantially two situations make it necessary to abandon the pipeline on the bed: bad weather conditions making laying work dangerous; and completion of the pipeline.
The pipeline must be recovered off the bed to resume laying and assembly work interrupted by bad weather.
Methods of abandoning and/or recovering underwater pipelines comprise shutting down assembly of the pipeline; connecting the free end of the pipeline to a hoisting assembly comprising at least one winch on the laying vessel, at least one rope, and a connecting device for connecting the rope to the pipeline; and abandoning/recovering the pipeline by winching out/up the rope. Documents EP 1,850,043 A2; US 2007/0177944; WO 2009/002142; and WO 2009/082191 describe various abandoning and/or recovery methods, which employ hoisting assemblies comprising two winches installed on board the laying vessel and operating synchronously to simultaneously control two ropes and share the load exchanged between the pipeline and the laying vessel.
The load between the laying vessel and the pipeline varies between a maximum and minimum, and depends on the length of pipeline raised off the bed. When abandoning the pipeline, load is maximum at the initial stage, when a long portion of the pipeline is raised off the bed. And, conversely, when recovering the pipeline, load is maximum at the final stage, when, again, a long portion of the pipeline is raised off the bed.
The load between the pipeline and the laying vessel, in fact, is a function of the weight per linear meter of the pipeline, and the length of pipeline raised off the bed.
Whether it comprises one or more winches, the hoisting assembly must have a total capacity greater than the actually short-lived maximum load between the pipeline and the laying vessel. Which means the laying vessel as a whole must be equipped with a hoisting assembly capable of hoisting more than the maximum load exchangeable between the pipeline and the vessel. Deepwater laying vessels, normally equipped with a J-lay tower, must therefore be equipped with hoisting assemblies of over 1500-ton capacity when working with extra-large-diameter pipelines; and even a 1500-ton hoisting capacity may not always be enough when abandoning extra-large-diameter pipelines in deep water.
To solve the problem, U.S. Pat. No. 6,729,802 filed by the Applicant proposes connecting the end of the pipeline to floating bodies to reduce the load between the pipeline and the laying vessel. This has proved highly successful, but is slow when the pipeline has to be abandoned rapidly in particularly bad weather conditions.